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Full text of "The bee Svastra sabinensis : nesting biology, mature oocyte, postdefecating larva, and association with Triepeolus penicilliferus (Apidae, Apinae, Eucerini and Nomadinae, Epeolini)"

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Number 3850, 12 pp. 

February 18, 2016 

The Bee Svastra sabinensis : Nesting Biology, Mature 
Oocyte, Postdefecating Larva, and Association 
with Triepeolus penicilliferus 
(Apidae: Apinae: Eucerini and Nomadinae: Epeolini) 



Information on the nesting biology of the large, ground-nesting, communal eucerine Svas¬ 
tra ( Epimelissodes ) sabinensis sabinensis (Cockerell) from Arizona is added to a previous 
account (Rozen, 1983). Details of nest size, location, depth, and structure are reported. Mature 
oocytes dissected from females are illustrated and described. Further information on its cocoon 
is presented and interpreted with respect to how it functions. The postdefecating larva is 
described and compared with that of S. o. obliqua (Say), the only other Svastra larva described 
to date. The association of this bee with Triepeolus penicilliferus (Brues) is further confirmed 
with the recovery of an immature larva of this cleptoparasite from one of the nest cells. 


While participating in a field trip on August 30 sponsored by Bee Course 2015, one of the 
students, Paige Muniz, discovered a number of large flying bees loudly buzzing as they circled 
around and entered a conspicuous hole in nearly level ground 4 miles east of Willcox, Cochise 
County, Arizona (figs. 1, 2). Shortly thereafter, she identified another similar open hole about a 
half meter away at the base of a low growing shrub where at least two more females of the same 
bee were emerging (fig. 3). A week later, Brice Lawley, volunteer at the Southwestern Research 

1 Division of Invertebrate Zoology, American Museum of Natural History, New York. 
Copyright © American Museum of Natural History 2016 

ISSN 0003-0082 



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FIGURES 1-7. Nesting site of Svastra s. sabinensis at 4 miles east of Willcox, Cochise Co., AZ. 1. Habitat 
picture showing B. Lawley at the nest. 2. Nest entrance (arrow) above pen when first discovered. 3. Students 
of Bee Course 2015 examining second nest entrance with first entrance identified by arrow. 4. Females emerg¬ 
ing from nest entrance (photo by R Muniz). 5. Partly exposed upper 15 cm of nest tunnel into which a slender 
straw had been inserted. 6. The author excavating the nest on the third day. 7. Inner surface of cell closure, 
showing spiral configuration. 




Station, was assisting the author and discovered yet another nest entrance of the same species at 
the base of a bush several meters away from the original discovery. This locality is well known to 
many entomologists as a place of considerable biological diversity. The soil consists of very fine, 
consolidated sand deposited there after being blown from the Willcox Playa to the southwest. 
This soil supports and is stabilized by vegetation consisting of a variety of plants including Pro- 
sopis, Psorothamnus, Euphorbia ( Chamaesyce ), Isocoma, Nama, and Verbasina. Although the bees 
were recognized as a species of Svastra, their specific identity was determined only after speci¬ 
mens were compared with those in the collection at the American Museum of Natural History; 
it was recognized as S. ( Epimelissodes ) sabinensis sabinensis (Cockerell), a species whose nesting 
biology was briefly reported earlier (Rozen, 1983). The discovery of this new site now offers a 
more complete understanding of its nesting behavior and allows the descriptions of its mature 
larva, the mature oocyte, and a functional interpretation of its cocoon. 

On the day of discovery at Willcox (August 30, 2015), numerous females were observed 
carrying pollen into the nest, and two female Triepeolus penicilliferus (Brues) were netted as 
they flew around the entrance. On the next visit a number of S. sabinensis females were 
observed emerging from the nest (fig. 4), and so on September 3 a plastic tumbler was inverted 
over the hole early in the day; later the same day 14 females of S. s. sabinensis were found 
captured in the tumbler when it was removed. Because no other females were encountered 
around the nest afterward, the captured females appeared to be the total nest population. They 
may have obtained their pollen from burro-weed ( Isocoma tenuisecta Greene, Asteraceae, det. 
Marilyn D. Loveless), which grew abundantly at the site. 

The nest entrance was open and lacked any indication of a tumulus. The open tunnel 
descending down from it was about 8 mm in diameter and completely unlined with any applied 
coating (fig. 5). It descended vertically for 30 cm, its lower part gently curving. Below that level, 
the tunnel began to divide dichotomously, in each case with both branches curving and con¬ 
tinuing to meander obliquely downward in some cases as far as 70 cm in depth and 70 cm 
horizontally from the nest entrance (fig. 8). Although only one side of the nest was excavated, 
some branches were seen to start to descend in the opposite direction, suggesting that the nest 
might have had a horizontal spread of at least 140 cm. 

No recently provisioned cells were encountered during the first two days of excavation 
(September 3 and 7). Two cells close together, each containing a postdefecating larva, were 
encountered on September 7, permitting subsequent examination of cocoon structure and 
function (see Cocoon Description, below) and larval anatomy (see Postdefecating Larva 
Description, below). Although orientation of the cells could not be determined because the 
orientation of the clod of substrate in which they were recovered was uncertain, cocoon frag¬ 
ments from the soil elsewhere indicated the long axes of cells were vertical with front ends up, 
as also reported by Rozen (1983). These two larvae almost certainly belonged to the previous 
generation that had failed to develop further unlike other members of their generation. They 
were still in complete diapause and the cocoon surface was old and fragile, whereas adults fly¬ 
ing about and provisioning were freshly emerged and without wing wear. 



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Although one cell from 
which a larva was recovered had 
been seriously damaged during 
excavation, the other cell was 
removed sufficiently intact to 
permit its description. It had an 
elongate oval shape 17.5 mm 
long and 8.5 mm in maximum 
diameter. The wall surface was 
remarkably smooth, and, 
although there was no visible 
lining, a water droplet placed on 
the surface beaded rather than 
being absorbed immediately. 
Thus, the surface had been ren¬ 
dered impervious to moisture. 
Some cell fragments from ear¬ 
lier generations displayed walls 
that were more consolidated 
than the surrounding soil, a fur¬ 
ther indication that the female 
had either mechanically consoli¬ 
dated (tamped) the soil or added 
some substance that did so. The entrance tunnel diameter at the closure was 7.0 mm. The 
closure plug itself (fig. 7) had a concave spiral inner surface consisting of about five coils, with 
an uncoated, rough texture. The entire closure plug was a soil cylinder about 10 mm long. Its 
outer end was smooth and evenly concave. The tunnel beyond was open. 

Just prior to silk production the larva started defecating by placing feces at the closure end 
of the cell, given that all fecal material was found only immediately under the spiral closure 
surface and on the upper end of the cell wall. Because the fecal mass was impregnated with 
silk, production of silk commenced during defecation. Because lower cocoon walls contained 
no feces, it can be assumed that cocoon construction continues well beyond the end of defeca¬ 
tion. The cocoon is described below under Cocoon Description. A strongly curled white larva 
in diapause with head partly hidden by the abdominal apex (fig. 15) was recovered from the 
bottom (posterior) end of each cocoon. 

On the third (final) day of nest excavation (September 9) by which time the excavation had 
become large (fig. 6), two cells containing feeding larvae were encountered. They were found 
at depth of 70 cm and 70 cm measured horizontally from a perpendicular line descending from 
the nest entrance hole (fig. 8). One contained a feeding intermediate stage larva of Triepeolus 
(identified to genus by slender, apically elongate, sclerotized mandibles and a pair of erect, 
forward projecting labral tubercles). It almost certainly was T. penicilliferus, since in addition 



FIGURE 8. Schematic illustration of dissected part of nest showing 
gentle curvature of main tunnel above and proliferation of meander¬ 
ing burrow tunnels below as they repeatedly divided, and spread 
down- and sideward. Approximate position of new and older occu¬ 
pied cells indicated. 




to the two female specimens of this cleptoparasite collected at the time of nest discovery, two 
more females were collected near the nest on subsequent days. The occupant of the other cell 
was a third or fourth larval instar of S. s. sabinensis, recognized by its two curved, apically 
twisted teeth on each mandible. Both cells contained remains of moist, fresh provisions. 

In overview, the nest appeared to be large in that the 14 females constructed tunnels that 
branched repeatedly, penetrating a considerable volume of ground. Since so few cells were encoun¬ 
tered, we can probably assume that tunnels on just the one side of the nest extend farther than our 
excavation. The few other nests observed during this three-day excavation seemingly had fewer 
females, so that the excavated nest may be unusually large (i.e., well occupied). As suggested by the 
remoteness of cells, it is likely that each female, after entering the nest, seeks out its particular branch 
and alone attends to the construction and provisioning of its cells at the lower end. If this is true, 
then the only nest part used communally by the group is the entrance tunnel. 

How each female when underground recognizes and finds its particular branch through 
the curving, meandering, and branching maze of the tunnel system will require further inves¬ 
tigation. We can assume the presence of so many Svastra females would be a deterrent for adult 
cleptoparasites in light of observed receptions of cleptoparasites by other adult hosts. However, 
one cleptoparasite, Triepeolus penicilliferus, seems to have successfully overcome such intolerant 
encounters, but how? We have more to learn! 

One approach in dealing with these challenges might involve selecting a study nest occu¬ 
pied by fewer cohabiting host females. Whatever size nest is selected, adequate nest-digging 
time should be allocated to explore the entire underground distribution of the nest tunnels. 
Although three working days were spent exploring the current nest, at most only one half of 
it was revealed and that included only a single series of current cells; considering the number 
of pollen-laden females, there must have been others extending farther out. 

Other questions arise as a result of the current investigations: The nest was being used by 
two generations as evidenced by uncovering mature diapausing larvae in cocoons from the 
previous year and observing simultaneously pollen-carrying fresh females returning to their 
cells in the same nest. There was considerable evidence of pieces of old nest cells among the 
massive amount of debris removed during the current study. To what extent does a single nest 
persist through the years, and/or to what extent does the female progeny of one nest establish 
new nests rather than return to their natal nest? 

In the final section of this paper (see Discussion), this study is compared with the early 
account of the biology of this species (Rozen, 1983) and with early biological investigations on 
other species of Svastra (Rau, 1922; Custer 1928, 1929; Rozen, 1964 2 ). 

2 In this paper, the egg of Svastra o. obliqua is depicted as it is undergoing eclosion (fig. 5). The interpretation 
presented was that at hatching, the shiny embryonic cuticle is first exposed on each side of the egg chorion 
along the spiracular line of the first. As the embryonic cuticle is sluffed off, it takes the remnants of the cuticle 
with it. There follows a total of four larval instars. However, a more recent interpretation documented in 
Rozen et al. (2011) with Centris flavofasciata Friese (Apinae: Centridini) is that the so-called embryonic 
cuticle is actually the cuticle of the first instar, and there is, therefore, a total of five larval instars, which likely 
will be found true for all bees. It follows that what is described as “first instar” of S. o. obliqua in Rozen (1964) 
is actually the second instar by this revised interpretation. 



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FIGURE 9. Diagram of mature oocyte, lateral 
view, showing curvature, rounded ends (anterior 
end to left). 

Figures 9-11 

Although no freshly deposited eggs were 
recovered from the nest, examination of the 
mature oocytes dissected from ovaries of females 
preserved in Kahles solution is informative. Three 
females had an ovarian formula of 4:4, as expected 
for most nonparasitic, solitary Apidae. The largest 
mature oocyte dissected from each of three pol¬ 
len-carrying females was strongly curved with both front and rear ends rounded (fig. 9). 
Although somewhat misshapen after being preserved in situ in the females’ body for about a 
month, each oocyte had a length of 3.0, 2.6, and 2.7 mm respectively and a maximum width 
of 0.65 mm toward the rear in all cases. The front end maximum widths were slightly less in 
all three. The chorion of each oocyte examined was poorly developed and thin, so that its 
sculpturing could not be determined. The micropyle on one (figs. 10, 11) was partly formed 
and seemed to be on a slight protuberance exhibiting a developing cluster of pores (fig. 11) on 
the front end. On another oocyte a single hole at the front end was interpreted as the start of 
micropylar development. 

Figures 12-14 

Cocoons containing fecal material at the upper end completely filled their cells, i.e., their 
shape is the shape of the cell lumen. The upper end of the cocoon is opaque viewed from the 
outside due to the incorporated fecal material. The lower end, however, is transparent dark 
brown, although this is hard to see without light being transmitted through. The cocoon wall 
consists of several layers of transparent smooth silk, which is highly reflective when viewed 
from the inside. However, the top of the cocoon on the inside is a nearly flat, thick circular disc 
of fine silk fibers, in one case with a diameter of 8.2 mm, which is opaque but reflects a soft 
sheen. Several thin, loosely attached sheet of transparent brown silk are found above the disc. 
They enclosed air spaces between the inner and outer surfaces of the cocoon top, as was dia¬ 
grammed for the related Svastra o. obliqua (Say) 3 by Rozen (1964: fig. 2). 

SEM examination of the cocoon confirmed the shiny cocoon wall is a sheet of silk. Even 
though it has a coating of fibers, it is without openings, an indication that it is airtight. The 
nearly flat circular disc of the top of the inside is indeed a thick mat of fine fibers, through 
which air can flow. SEM examination of the brown sheet silk above the disc was inconclusive 

3 The surface of the inner layer of the cocoon top of Svastra o. obliqua is depicted as somewhat domelike in 
Rozen (1964: fig. 2, termed “roof” therein), rather than as a nearly flat disc as in the case of S. s. sabinensis. 
This distortion in the early study may have been caused by body pressure from the larva or pupa within the 
cocoon lumen below. 




FIGURES 10-14. 10. SEM image of front end of oocyte of Svastra s. sabinensis showing position of micropyle 
(arrow). 11. Same, close-up of developing micropyle. 12-14. SEM images of cocoon fabric. 12. Cocoon wall, 
showing lack of fenestration in fabric. 13. Cocoon disc, inner view, showing ample passageways through dense 
fibers. 14. Same, close-up of cut edge. 

as to whether air exchange was in spaces around these sheets through the fecal layer above or 
through spaces along the upper sides of the cocoon. 

Figures 15-21 

Although alive, the two tightly curled postdefecating larvae retrieved were flaccid and 
badly misshapen, presumably because of loss of fat tissue and water, and therefore were difficult 
to illustrate. The distortion of body mass is further evidence that the specimens represented 
the previous generation. 

The descriptive anatomy of the mandible of a eucerine bee larva used here follows Rozen 
(1965, 1991) and Michelette et al. (2000). 

Diagnosis: The strong dorsolateral swellings of most caudal annulets on both sides of the 
larva give it a strong robust appearance. These swellings, however, are not pronounced when 


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FIGURES 15, 16. Diagrams of postdefecating larva of Svastra s. sabinensis. 15. Entire larva in diapausing 
posture, lateral view. 16. Head, frontal view. 

the larva is viewed in lateral profile (fig. 15). Although the larva of Svastra s. sabinensis described 
here appears to be somewhat more sclerotized than specimens of S. o. obliqua preserved from 
collections described by Rozen (1964), no other differences could be detected. 

Description: Head (fig. 16): Integument with a few scattered fine setae. Except for darkly 
pigmented mandibular apex, natural pigmentation at most faint. Fine, dense spiculation restricted 
to apex of labrum and epipharyngeal surface. Coronal ridge well developed, extending from 
postoccipital ridge to epistomal ridge; postoccipital ridge well developed, extending nearly directly 
across cranium as seen from above, scarcely curving forward at median line; hypostomal ridge 
well developed; dorsal ramus not developed: epistomal ridge present laterad of (below) anterior 
tentorial pits and between pits, its ventral edge between pits well defined, its dorsal edge grading 
into more heavily sclerotized frontal area; tentorium moderately robust including dorsal arms. 
Parietal bands evident but not pronounced. Antennal prominence weakly developed and cranium 
above prominence evenly curved in lateral profile; antennal papilla small, shorter than basal 
diameter, and bearing about three sensilla. Clypeus, labrum, and labial apex broad in frontal view 
(fig. 16). Labral apex bearing pair of tubercles widely separated along lower edge (fig. 16). 

Mandible (figs. 17-19) robust in dorsal and ventral views; mandibular apex becoming 
slender in inner view with extreme apex with two flattened, apically pointed and twisted teeth, 
dorsal one of which is distinctly longer. Apical concavity directed ventrally because dorsal edge 
of concavity directed much farther adorally than ventral edge; dorsal inner edge of concavity 
closely paralleled by inner ridge (fig. 18, ridge); narrow surface between this edge and inner 
edge, termed dorsal plane, denticulate; these denticles 4 (fig 19, denticles) becoming long, seta- 

4 Although elongate and curved, the denticles are not setae as seen in close-up view for they do not rise from 




FIGURES 17-21. Macrophotographs of right mandible (with setae on outer edge accidentally removed), in 
17. dorsal, 18. inner, 19. ventral views, respectively, 20, 21. Macrophotographs of spiracle of cleared specimen, 
showing fine spicules, pigmentation, and shallow atrium, in 20. outer and 21. side views, respectively. 

like in vicinity of the-cusp where they form distinct brush and where inner ridge becomes 
lamelliform; mandible apically bidentate with dorsal tooth longer than ventral tooth; outer 
surface of mandible with three conspicuous setae arising near base (accidentally destroyed 
during dissection). Cardo and stipital rod sclerotized but not extensively pigmented; articulat¬ 
ing arm of stipes evident; maxilla narrowing to moderately pointed apex directed mesad 
beyond insertion of maxillary palpus, so the palpus is subapical in position; galea distinct 
mound, bearing sensilla; maxillary palpus cone shaped, length about twice basal diameter. 
Labium clearly divided into prementum and postmentum, wide (fig. 16); premental sclerite 
weakly sclerotized, labial palpus moderately slender, length about three times basal diameter, 



NO. 3850 

shorter than maxillary palpus. Salivary lips projecting, widely transverse because of breadth of 
prelabium, width about equal to distance between bases of widely separated labial palpi. Hypo- 
pharynx with flat surface. 

Body (figs. 15, 20, 21): Body entirely lacking vestiture of setae and spicules (though with 
nonsetose sensilla on abdominal segment 10); body surface of postdefecating form strongly 
wrinkled. Body form of postdefecating larva robust (fig. 15); caudal annulets of thoracic seg¬ 
ments and of abdominal segments 1 and 2 with dorsolateral swellings on each side of midline; 
these swellings on subsequent abdominal segments not developed. Abdominal segment 10 
attached to approximate middle of segment 9; anus presumably positioned toward top of seg¬ 
ment 10. 5 Spiracles (figs. 20, 21) well sclerotized, large, distinctly pigmented, subequal in diam¬ 
eter; atrium extremely shallow, with diameter more than twice depth, not projecting much 
beyond body wall, presumably with rim expressed; subatrium short, consisting of about four 
chambers; both atrium and subatrium very finely spiculate. Sex-specific characters unknown. 

Material studied: Two postdefecating larvae: USA: AZ: Cochise Co.: 4 mi E. of Willcox, 
7 Sept. 2015 (J.G. Rozen, B. Lawley). 


The only previous account of the biology of Svastra s. sabinensis (Rozen, 1983) described 
and pictured a similar nest from near Tucson, AZ, which was also occupied by more than one 
female. The one significant difference from the current nest was the occurrence of pebbles (see 
figs. 2, 3, therein) clustered in the main shaft and side tunnels, then a puzzling observation, 
which can now be explained. Oddly the explanation is derived at least in part from a recent 
study of a wood nesting bee, Lithurgus chrysurus Fonscolombe (Rozen, 2013), in which it was 
suggested that blind tunnels, chambers, and antechambers found in many nests may often be 
a source of materials to block tunnels leading to recently closed cells, securing them from para¬ 
sites and predators. This suggests that the pebbles in figure 3 from the 1983 study were cumber¬ 
some residue left behind by the female when she quarried soil to fill the tunnel to her recently 
constructed, provisioned, and closed cell, after egg deposition. Figure 2 therein demonstrates 
the heavy load of pebbles in the soil at the site near Tucson. In contrast, fine soil at the current 
site lacked pebbles, as did nest tunnels. 

The only other investigations on the biology and immatures of the genus Svastra are those 
of Custer (1928, 1929) and Rozen (1964) of S. o. obliqua. Custer (1928), who assigned the spe¬ 
cies to Melissodes, recognized that many nests were occupied by more than one female, identi¬ 
fied Triepeolus concavus Cresson as its possible cleptoparasite, and presented diagrams of a nest 
(showing antechamber) and a close-up of the cell containing provisions and a small larva. 

5 Because of the close overall agreement between the mature larvae of Svastra s. sabinensis and S. o. obliqua, 
the predefecating larva can be relied upon to interpret certain features of larval anatomy of S. s. sabinensis. 
Postdefecating larvae of both species recovered strongly curled so that heads deformed the abdominal apices. 
As demonstrated for S. o. obliqua (Rozen, 1964: fig. 7), abdominal segment 10 has the anus positioned on a 
terminal projection toward the top of abdominal segment 10. Thus, the prediction for S. s. sabinensis. 




Interestingly, in one case he found a nest entrance that was being used simultaneously by a 
smaller species of “Melissodes.” 

In his subsequent paper, Custer (1929) concluded that the Triepeolus was “carnivorous” 
Although 29 of the total 30 cleptoparasites died before maturing, all were in cocoons identical 
to those spun by S. o. obliqua. He concluded, therefore, that the Triepeolus larva feeds on the 
cocoon-covered host larva. We now know that no larva of any Epeolini has been found to feed 
on the mature host larva. His statement that “the parasitic larvae possess two rows of dorsal 
tubercles which are more highly developed than in the case of the host” does not correspond 
to current descriptions of Triepeolus larvae (Michener, 1953; Rozen, 1966). None possess paired 
dorsal tubercles, and all are cleptoparasites whose larvae feed on the provisions stored by the 
adult host, the egg or young larva of which is killed by the first larval instar of the cleptopara- 
site. First instars of Triepeolus have hospicidal anatomies (slender body, heavily sclerotized and 
pigmented head capsules, extremely elongate mandibles, elongate and forward-projecting 
labral tubercles; Rozen, 1989).The source of Custers misunderstanding is not understood. 

Rozens (1964) account of the larva and nesting biology of Svastra o. obliqua demonstrates 
a close agreement with the feature described above for S. s. sabinensis. The only difference 
detected between the two was that the nest of S. o. obliqua as measured by cell depth was 10-25 
cm, contrasting with 70 cm (current study) and 80+ cm (Rozen, 1983) for S. s. sabinensis. 

It is interesting to note that the circular shape of the postdefecating larva (with head embed¬ 
ded in abdominal segment 10) that seems characteristic of both Svastra sabinensis and S. obliqua 
is not unique to them. Postdefecating larvae of the eucerines Xenoglossa angustior Cockerell 
(Rozen, 1965: fig. 1) and Canephorula apiformis (Friese) (Michelette et al., 2000: fig. 22) are also 
circular, a posture probably dictated by a long diapause at the bottom of a vertical cocoon. 


I thank Paige Muniz for calling me over to the interesting hole in the desert floor into 
which these bees were disappearing, and I extend additional thanks to her fellow students of 
Bee Course 2015 for their assistance at the site looking for further nests of this and other bee 
species. Brice Eawley, volunteer at the Southwestern Research Station, vigorously assisted me 
during the three days we excavated this single nest. 

Corey Smith, Curatorial Assistant, AMNH, assisted in the discovery and early exploration 
of the nest in Arizona, by preparing the SEM images used here, and by proofreading the manu¬ 
script. Steve Thurston, Senior Scientific Assistant, AMNH, prepared figure 8 and arranged all 
plates for the resulting publication. 

The Southwestern Research Station of the American Museum of Natural History has over 
a period of 17 years well served the Bee Course, the annual workshop for training biologists 
and other professionals worldwide who need to learn more about bees. Further, I acknowledge 
the central role that this sophisticated field facility has played over a 55-year period on my 
research dealing with the rich fauna of these animals in southern Arizona and New Mexico. 

Lastly, my appreciation is extended to two outside reviewers for their valuable comments 
and suggestions. 



NO. 3850 


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